Seal materials for vessel caps that contain polyvinyl chloride (PVC) have long been used in the packaging industry.
The use of PVC-containing compositions in packaging materials is generally no longer desirable however for many reasons. When household waste is incinerated, acidic gases are produced from halogen plastics and the escape of said gases into the atmosphere is harmful. In addition, even small amounts of PVC interfere with the material recycling of plastic waste. Furthermore, PVC-based seal elements require the use of plasticizers, which may potentially migrate into the food contained in the vessel and are therefore questionable for health reasons.
The object of the present invention is to provide a PVC-free composition (also referred to hereinafter as a polymer compound) for vessel caps, in particular for the packaging of foods. Foods (including beverages such as juices and the like) are often packaged in vessels made of glass or plastic, which in many cases then have a screw lid. The term “screw lid” is representative here for vessel caps that, in the filled and closed state, are engaged with the vessel by means of a thread. To open the vessel, the cap has to be rotated relative to the vessel, wherein the seal of the cap lifts from the vessel edge and the vacuum (often) provided in the vessel is cancelled. The cap can be separated from the vessel by such a rotation. The known PVC-containing seal materials have the processing and performance characteristics necessary for this purpose. It should thus be noted that a PVC-free sealing compound is only then a seal material of commercial interest for vessel caps if the PVC-free sealing compound has quite specific physical-chemical properties not inferior to those of PVC-containing seal materials.
Hooded lids (for example for mustard jars) or single-use lids, for example crimp-on lids, which are used on glass containers having large diameters, are likewise caps within the meaning of this invention.
More specific requirements for example include the following aspects:                The material composition is to be selected such that undesirable substances are avoided. The sealing compound should therefore not contain substances that are classified as presenting a health risk, in particular plasticizers, such as phthalates; semicarbazide and sources thereof, in particular ADC and OBSH; 2-ethylhexanoic acid and sources thereof; organic tin compounds, primary aromatic amines, bisphenols, nonylphenol; BADGE; photoinitiators; perhalogenated compounds; melamine.        For some applications, the presence of larger contents of liquid substances (that is to say substances that are liquid at RT) is undesirable. Then, the content of such substances (such as white oil) should be at most 10%, preferably less restricted, and in some cases the sealing compound should have no traceable contents of such liquid substances.        If bisphenol-A and melamine are to be avoided, coatings that do not contain such substances are used for the vessel cap. The seal material should then be composed such that a lasting adhesive bond is achieved with such coatings.        The material composition should be selected such that the seal material satisfies even challenging requirements during use.        The seal material should thus preferably be usable under pasteurization or even sterilization conditions, that is to say should withstand a pasteurization (up to 98° C.) or a sterilization (generally above 100° C., often above 105° C. or above 110° C., or even above 120°, up to 132° C.).        For some uses, the seal material should have a barrier function, that is to say should reduce or prevent the infiltration of undesirable substances into the vessel.        For specific applications, it should be possible to provide the seal material with absorbing additives (for example oxygen absorbers) or scavenger substances.        The seal material must have the required processing characteristics.        In principle, it must soften thermally to a sufficient extent so as to be useable on conventional processing machines (in particular for extrusion with subsequent stamping or compression molding or in injection molding methods).        It must therefore have the necessary sealing properties after introduction into the vessel cap and cooling to the desired application temperature (generally RT, but possibly also at lower temperatures, for example in a chilling cabinet).        It must also be possible to introduce the seal material over the entire area for small vessel caps.        For PT caps (Press-on Twist-Off® caps), the seal material must form both the seal and the inner thread of the cap, and it must therefore be possible to apply the seal material (as what is known as a “contoured ring”) both to the inner face and to the skirt of the cap, and the seal material must also be able to form the thread elements when the cap is pressed on.        For some applications, the seal material should be able to form the seal insert “out shell”, that is to say outside the cap, the seat insert then being inserted as a finished ring seal or the like into the vessel cap.        The seal material is to be suitable in particular for metal caps and metal-plastic composites that may be coated on the inner face, however it is also to be suitable for plastic caps.        The seal material must be suitable for conventional food packagings.        The seal insert must be suitable for pasteurization (up to 98° C. or more) and should preferably also be suitable for sterilization (up to 132° C.).        The seal insert must withstand a post-treatment (pasteurization and the like) at counterpressure and evacuation; if necessary, it must have vacuum retention and barrier properties where applicable.        The seal insert is to be suitable for conventional vessels made of metal, plastic, glass, etc.        In the event of contact with the filled content, in particular fat-containing foods, alcoholic beverages and other products of lipophilic nature (compared to water), the seal insert is to deliver no components or minimal components to the filled content.        The seal insert must demonstrate sufficiently low twist-off values in order to be able to remove the vessel cap (possibly with cooling) with moderate force. At the same time, the seal must demonstrate its seal insert effect over the intended lifetime (minimum shelf life) of the food.        
The objects addressed by the present invention are achieved by the PVC-free compositions defined in the independent claims. Advantageous embodiments are defined in dependent claims.